Contamination of ground water by volatile and semi-volatile organic compounds is a widespread and well documented problem. Such contaminants enter the ground water from various sources such as underground storage tanks, municipal and industrial landfills, and industrial effluents. The contaminants may also enter the water as unintended by-product of conventional chemical treatment processes utilized to disinfect the water.
As the concentration of contaminants in ground water approach or exceed "acceptable levels", the contaminants must be removed to render the water potable. Hence, as the water supply used by a municipality and/or private well owner approaches the "acceptable level" for a contaminant, the municipality and/or private well owner must either accept the risk associated with such levels of contaminant, locate an alternative water source, or implement a treatment processes for removing the contaminant. Generally, treatment processes for removing contaminants from water are extremely expensive.
The most common method of removing contaminants from water is to contact the water with granular activated carbon. Treatment with activated carbon is generally the treatment of choice because it can readily remove a wide variety of typical contaminants. However, while activated carbon is effective at removing the less volatile contaminants such as PCBs, PAHs, and phenolics it is not particularly effective at removing the more volatile contaminants such as chloroform, 1,1,2-trichloroethane and trichloroethylene because of its low affinity for such contaminants. Hence, effective use of activated carbon to treat water contaminated with a volatile contaminant requires frequent replacement of the activated carbon to maintain optimum affinity of the activated carbon for the contaminant. Such frequent replacement of the activated carbon can significantly increases the cost of an already expensive process.
A second commonly employed method of removing contaminants from water is to pass the water through an air stripping tower. Basically, an air stripping tower removes contaminants from water by cascading the water over a packing material designed to uniformly disperse the water throughout the tower while providing an upward flow of air which is also designed to uniformly disperse the water throughout the tower as well as provide a supply of air into which the contaminants may dissipate. However, effective operation of air stripping towers is difficult as they are readily susceptible to flow channeling and flooding.
Contaminants may also be air stripped from water without encountering the flooding and flow channeling problems associated with air stripping towers by using a hollow fiber membrane contactor. Hollow fiber membrane contactors remove contaminants from water by passing the contaminated water through the lumen of microporous, hydrophobic, hollow fiber membranes while passing air over the outside of the hollow fiber membranes. The hydrophobic nature of the hollow fiber membranes retains the water within the lumen of the fiber while the fiber micropores permit diffusion of the contaminants through the fiber and into the air.
Air stripping is the treatment method of choice for removing volatile contaminants from water because of its relatively low cost. However, in order to prevent contamination of the atmosphere with the stripped contaminants it is typically necessary to recover the contaminant from the air prior to its release into the atmosphere and such secondary recovery can significantly increase the cost of the treatment. In addition, air stripping is not particularly effective at removing semivolatile and non-volatile contaminants as such contaminants are not readily volatilized from the water into the air.
The drawbacks associated with the processes commonly employed to remove contaminants from water has resulted in a continued need for an inexpensive alternative technique for achieving the effective removal of contaminants, particularly volatile contaminants, from groundwater.